Start-up system for once-through boiler



Oct. 6, 1970 A. J.-ZIPAY ETAL START-UP SYSTEM FOR ONCE-THROUGH BOILERFiled July 2, 1968 SUPERHTR.

EMINERLZR Low PRESS.

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IXI-ENTOR. ALBERT 3. ZlPAY JUSTIN EWINKIN ATTOKNEY5 Patented Oct. 6,1970 3,532,453 START-UP SYSTEM FOR ONCE-THROUGH BOILER Albert J. Zipay,Clifton, and Justin P. Winkin, Fairlawn,

N.J., assignors to Foster Wheeler Corporation, Livingston, N .J acorporation of New York Filed July 2, 1968, Ser. No. 741,964 Int. Cl.F22b 29/06 US. Cl. 122-406 13 Claims ABSTRACT OF THE DISCLOSURE Aonce-through vapor generator start-up system of the type which includesa generator main flow path having in series heating surfaces, acondenser, and one or more feedwater heaters. A constant pressurestart-up bypass line extends between the heating surfaces and thecondenser and feedwater heaters, comprising a flash tank and first andsecond drain connections leading from the flash tank to the feedwaterheaters and condenser respectively. A valve in the second connection tothe condenser controls the drain flow to the condenser in response tomaximum and minimum water levels in the flash tank. At levelsintermediate the maximum and minimum levels, the flow in the secondconnection is controlled in response to the flow in the first connectionto the feedwater heaters.

DESCRIPTION This invention relates to apparatus and a method forstarting up a once-through vapor generator, particularly of the typewhich includes a bypass line between heating surfaces of the generatorand heat recovery surfaces thereof.

It is known to provide a bypass line in a once-through vapor generatorto bypass the flow in the generator around the unit turbine. The reasonfor this is that during the start-up period for the generator, thequality of the fluid is not such that it can be handled by the tur bine.Generally, the bypass line includes a flask tank, with a connection toreturn vapor flashed in the flash tank to the generator main flow pathfor early starting of the generator turbine. The bypass line and flashtank may also be provided with connections between the vapor space andliquid space of the flash tank and heat recovery components, such as agenerator feedwater heater and deaerator. Surplus flash tank heat may bedischarged to a heat sump such as a condenser through vapor or liquidline connections between the flash tank and the sump.

Normally the exhaust flow from the feedwater heater, which is of theshell and tube type, is valved in a way to produce a desired pressure inthe bypass line and at the turbine throttle during the start-up period.This can be done by controlling the liquid level in the feedwater heaterand in that way the amount of condensing surface in the heater which isuncovered. A predetermined amount of surface condenses the flow to theheater, either liquid flow flashed in the heater or vapor flow, and asthe level drops because of insufficient condensing surface, the exhaustflow valve from the heater is closed to raise the level. This increasesthe presssures in the heater and correspondingly the pressure upstreamof the heater. By controlling the level in the heater, the desiredpressure in the bypass system and at the turbine throttle can beobtained.

In the present invention, it is contemplated that the bypass line andfeedwater heater will be operated during start-up at a constantpressure, for instance 1100 p.s.i., holding turbine throttle pressureconstant at, for instance, about 1000 p.s.i.

The primary problem in such a pressure control for start-up is that itdoes not protect the feedwater heater from excessive flow from the flashtank. Normally, a generator of today will be provided with at least twohigh pressure heaters in parallel, each being designed or sized for halfof normal load extraction steam rates with some margin of safety. Abypass flow during start-up can be much higher, as high as 25% to 30% offull load flow, and if one of the heaters is out of service, and theother is required to accommodate the full 25 to 30% flow, the resultanthigh velocity flow in the heater can damage the subcooling section ofthe heater. The heater exhaust flow valve is not operative to protectthe heater since it will merely pass the additional flow to maintain apredetermined level. This condition, plus the situation where bothheaters may be out of service, requires a bypass around the heaters.Such a bypass also is necessary early in the start-up period, prior toburner light off, when the flow to the generator is heated by peggingthe deaerator. A bypass flow from the flash tank to the heaters wouldonly cool the feedwater.

The sump connection between the flash tank and a sump such as thecondenser provides such a bypass, this usually is valved to controlwater level in the flash tank, between minimum and maximum levels, andobviously would not suffice to control flow to the heaters whether one,two, or no heaters were in service.

It is an object of the invention to provide and improved control ofdrain flow from the flash tank to the feedwater heaters.

It is also an object of the invention to obtain automatically themaximum possible recovery of heat during the start-up period.

In accordance with the invention there is provided, for a once-throughvapor generator which includes a main flow path including heatingsurfaces therein, condensing means and feedwater heating means also insaid main flow path, a bypass line between said heating surfaces andsaid condensing means and feedwater heating means, the bypass lineincluding flash tank means and drain connections between said flash tankmeans and condensing feedwater heating means, the improvement comprisingcontrolling the flow in the connection leading to said condensing meansin response to maximum and minimum levels in said flash tank means. Atlevels intermediate said maximum and minimum levels, the flow in saidconnection leading to said condensing means is controlled in response toflow in the connection leading to said feed water heating means.

The invention and advantages thereof will become apparent upon furtherconsideration of the specification, with reference to the accompanyingdrawing, in which the figure represents schematically a flow arrangementand start-up system for a once-through vapor generator in accordancewith the invention.

Referring to the figure, the once-through vapor generator comprises amain flow path which includes an economizer 12, furnace passes 14receiving a fluid from the economizer, a primary superheater 16downstream of the furnace passes, and a finishing superheater 18downstream of the primary superheater. A high pressure turbine 20receives fluid from the finishing superheater 18, and the exhaust flowfrom the high pressure turbine 20 is reheated in reheater 22 and thentransmitted to a low pressure turbine 24. From the latter, the exhaustflow passes to condenser 26. Between condenser 26 and the economizer 12,the main flow path includes demineralizer 28 receiving flow from thecondenser, a low pressure heater 30, and a deaerator 32, the lattertransmitting the flow to feedwater pump 34. In the feedwater pump 34,the fluid, which can now be considered the feed flow for the generator,is pressurized, and transmitted to the tube sides of high pressureheaters 36-40 inclusive, and from there to the ecohomizer 12. The highpressure heaters 36 and 38 are in parallel with the heaters 40 and 42,and either set or both sets of heaters can be used. On the shell side ofthe heaters, the drain flows from the nos. 1A and 1B heaters (40 and 38)via lines 38a and 40a to the nos. 2A and 2B heaters (42 and 36), andfrom there to the deaerator 32.

Between the outlet of the primary superheater 16 and the heat recoveryand heat sump portions of the main flow path, which for purposes of thisinvention, will be considered those portions of the path including thecondenser 26 and high pressure heaters 36-42, is a bypass line meansgenerally designated by the number 44. The bypass line means primarilycomprises a conduit 46 connected to the outlet of the primarysuperheater leading to a flash tank 48. The flash tank in turn isprovided with a number of conduits leading therefrom, including a steamconduit 50 adapted to return a vapor flow from the vapor space of theflash tank to the inlet of the finishing superheater, and a drain line52 leading from the liquid space of the flash tank. The vapor line '50is provided with branch conduits 50a leading to the condenser, 50bleading to the turbine gland seal, and 50c leading to the deaerator, thelatter being for the purpose of pegging the deaerator. The drain line 52is provided with a branch conduit 52a leading to the condenser, and asecond branch conduit 52b leading to the high pressure heaters. Alsoleading from the flash tank is conduit 54, from the vapor space thereofto the high pressure heaters.

In the conduit 46 leading from the outlet of the primary superheater tothe flash tank is a shut-off valve 56, and in the conduit 50 leadingfrom the flash tank to the inlet of the finishing superheater, there ispositioned valve 58, both valves functioning to isolate the bypasssystem from the main flow path of the generator during normal loadoperation of the generator. During use of the bypass system, remotemanual control valve 60* in the main flow path between the primarysuperheater outlet and finishing superheater inlet can be closed todirect the flow through the bypass.

Also of interest with respect to the bypass system is pressure reducingvalve means 62 disposed in the main flow path between the furnace passesand primary superheater designed to reduce the pressure in the generatorduring start-up. As an example in accordance with the invention, thefeedwater pump may pressurize the generator furnace passes during thestart-up period to normal operating pressure, Which may be subcriticalor supercritical. The high pressure in the furnace passes is necessaryfor protection of these passes. To obtain a steam flow to the turbineearlier in the start-up period, the pressure reducing valve 62 reducesthe pressure at the outlet of the furnace passes to about 1100 p.s.i.,the steam at the lower pressure flashing in flash tank 48 earlier in thestart-up period. Reducing the pressure upstream of the primarysuperheater has the advantage of improved heat transfer at lowerpressure in the primary superheater, and thus additional heat pick-upand a shorter starting-up period for the working fluid in the generator.

The present invention is concerned primarily with the portion of thestart-up period in which there is a liquid or liquid and vapor flow intothe flash tank, and a liquid level established in the flash tank. Forthis period, the branch line 52b is provided with connections 52b and52b" to Nos. 1A and 1B high pressure heaters 40 and 38, respectively,each having a shut-off or isolation valve, items 64 and 64'respectively, so that sets of heaters, 40 and 42, or 36 and 38, can beused individually or in paral lel, or both sets of heaters can be out ofservice.

In the situation where none of the high pressure heaters is in service,all of the drain flow from the flash tank would have to be dumpedthrough line 52a to the condenser. Alternatively, where only one of thesets of high pressure heaters is in service, the other being out ofserv- 4 ice, the one in service is still incapable of handling all ofthe start-up flow, in which case some of the drain flow from the flashtank would have to be dumped to the condenser.

A particular example of the situation where no feedwater heater is inservice, is during the very early stages of the start-up period. Thedeaerator 32 may be pegged with steam from a separate or auxiliarysource, even prior to firing the burners, resulting in a heated flow offeedwater from the deaerator through the high pressure heaters, on thetube side thereof, to the economizer. The flow in the flash tank isstill cold and unless the heaters are isolated from the flash tank (byvalves 64 and 64) the undesirable condition of extraction of heat fromthe feedwater will exist.

A further reason for keeping the high pressure heaters out of serviceduring the very early stages of start-up is to isolate the heaters(using valves 64 and 64') until flash tank drain temperature reaches avalue that will subsequently flash steam in the heaters to produce thenecessary pressure to force a flow to the condenser.

To explain this in more detail, the heaters, as mentioned above, are ofthe shell and tube type with means for passing the feedwater through theheater tubes, and for introducing the bypass flow to the heater shellside. Usually the exhaust port for the heater shell-side flow is in thesubcooling section of the heater. Referring to the figure, the exhaustlines from the No. 1 heaters are provided with flow control valves 66and 66 actuated in response to liquid level in the heaters. During theearly stages of startup, there is insufficient pressure in the flashtank and/or in the high pressure heaters to force the fluid from theheaters to the condenser. The only way the flow can be forced to thecondenser may be to flood the heater and possibly the flash tank, andthis may be considered undesirable in certain installations.

Various reasons well known to those skilled in the art may exist why asingle heater may or may not be in service. For instance, it may be outof service for repair work. In any event variable amounts of fluid willbe diverted through conduits or connections 52a and 52b depending uponthe number of heaters in service and the point in the start-up period.Automatically controlling this is achieved in the following way.

The sump conduit or connection 52a, which in essence is a bypassconnection around the heaters, lead from the flash tank to the condenseror sump 26, and is provided with a flow control valve 68 actuated byflow controller 70. As shown, the flow controller receives maximum andminimum level signals from the flash tank and actuates the valve 68 sothat the liquid level in the flash tank is main tained between maximumand minium points. As the level in the flash tank drops below a minimumlevel, the flow controller 70 closes the valve 68 raising the level inthe flash tank, and conversely, as the water level in the flash tankexceeds the maximum level point, the flow controller 68 opens the valvelowering the level. The purpose of this is to provide a pressure sealagainst steam flow or leakage of steam through the conduit 52a to thecondenser. Also, depending upon flash tank design, a minimum level maybe necessary to obtain separation of vapor and liquid in the flash tank.By the same token, there is usually a maximum liquid level in the flashtank which must not be exceeded if flashing and separation is to occur.

In the conduit 52b, the start-up system is provided with a flow meter72, adapted also to transmit a signal to the flow controller 70. As longas the level in the flash tank is between maximum and minimum points,the flow controller is responsive to the signal from the flow meter 72.The controller 70 has three set points for this signal, one when allheaters are in operation, another when only one set of heaters is inoperation, and the third when no heaters are in operation. These setpoints are determined for maximum permissible heater flow capacity toobtain maximum heat recovery and still avoid damage to the heaters.Initially during start-up, in the very early stages of the start-upperiod, there will be no flow to the high pressure heaters, and the setpoint will cause the controller 70 to divert all of the drain flow tocondenser 26. If only one of the high pressure heaters is subsequentlyplaced in service, the second said point will adjust the flow in conduit52b to a predetermined amount within design limits of this heater, theflow in conduit 52a increasing or decreasing only if maximum or minimumlevel signals are received. When both high pressure heaters are inservice, all of the drain flow will normally pass to the high pressureheaters by virtue of the first set point for the flow controller.However, the minimum level set point in the flash tank is such that thesignal from the flow meter will be locked-out if the level drops belowminimum with the level signal then taking over and maintaining minimumlevel in the flash tank, or at least completely closing the valve 68 inline 52a. For instance in the later stages of start-up when the unit isat equilibrium and flash tank drain flow approaches zero, the steam flowthrough the flow meter could reach a magnitude calling for opening ofvalve 68 and dumping of valuable heat to the condenser. To prevent thispossibility, it is necessary to lockout the flow controllers effect onthe positioning of the valve 68.

EXAMPLE As an example in accordance with the invention, the generator isstarted with a pumping rate of about 25% of full load flow. The burnersare placed in service and fired at a rate of approximately of full loadfiring rate, subject to a minimum fuel limit for combustion stabilityand to a furnace exit maximum gas temperature of about 1l00. The controlvalves 62 hold fluid pressure within the furnace passes at about 3550p.s.i.g., in a supercritical unit, and valves in the bypass system andat the turbine inlet hold pressure downstream of the furnace passes atabout 1100 p.s.ig. The control loops in the bypass system automaticallydistribute the flow of water and subsequently the flow of steam from theflash tank for maximum heat recovery. This is accomplished as fol lows:

Initially the drain line valves 64, 64' and 68 are closed. All otheroutflow valves connected to the flash tank are closed allowing the waterlevel to build up in the flash tank. When the fluid temperature leavingthe furnace circuit reaches about 200 F., the minimum fluid temperatureto obtain flashing in the high pressure heaters, the valves 64, 64', 66,and 66' are opened and the high pressure heaters are placed in service.Depending upon whether one, two or no heaters are in service, the flowcontroller regulates the opening of valve 68 to maintain both aprescribed maximum permissible heater drain flow and flash tank levelbetween maximum and minimum points. With both high pressure heaters inservice, the flow controller 70 will close control valve 68 and all theflash tank drain flow will usually be to the heaters. With one highpressure heater in service, the flow controller will regulate theclosure of valve 68 until the maximum permissible drain flow to theheater for maximum heat recovery is obtained.

The admission of hot water to the high pressure heater or heaters, ifboth are in service, will result in flashing of steam and theestablishment of a pressure suitable for forcing the heater drain flowto the condenser. In the event insufficient pressure is available topass all of the drain flow, the flash tank level will rise above themaximum level set point, and valve 68 will open to dump or bypass aportion of the flow to the condenser. However, the level controllers inthe high pressure heater drain lines are set to control heater level orheater condensing surface at a point that will subsequently oreventually permit heater pressure and flash tank pressure to reachapproximately 1100 p.s.i.g., pressure upstream of the heaters being afunction of heater level or condensing surface with a predeterminedheater drain flow. During this stage of start-up, the turbine can beappropriately loaded.

With one heater in service, flash tank drain flow will be distributed toboth the heater and the condenser until the flash tank drain flowdecreases (because of increased flashed steam supplied to the turbineand other sub-loops) to a flow equal to the maximum required by theheater. When this occurs, control valve 68 will be closed and all of thedrain flow will be to the heater. This continues to maintain a pressureseal against vapor flow to the condenser assured by lock-out of anysignal from flow meter 72 which might tend to cause an opening at valve68.

Among advantages of the invention is the advantage that the heater ofheaters receive the maximum permissible flow for optimum heat recovery,whether one or all heaters are in service, bleeding to a heat sump onlythe amount of flow necessary to protect the heaters. At the same time,the bleed flow to the heat sump is controlled to maintain a desiredliquid level in the flash tank to pressure seal the tank against loss ofsteam to the sump, and for effective flash tank performance.

Although the invention has been described with respect to specificembodiments, variations within the scope of the following claims will beapparent to those skilled in the art.

What is claimed is:

1. A method for starting up a once-through vapor generator of the typecomprising a main flow path including heating surfaces, condenser meansand feedwater heater means in series, a bypass line between said heatingsurfaces and said condenser and feedwater heater means, including flashtank means and drain conduit means between said flash tank means andcondenser and feedwater heater means, comprising the steps of:

controlling the flow in said drain conduit means to said condenser meansin response to maximum and minimum levels in said flash tank means; and

at levels intermediate said maximum and minimum levels controlling theflow in said drain conduit means to said condenser means in response toflow to said feedwater heater means.

2. A method according to claim 1 wherein said feedwater heater meansincludes at least two high pressure heaters in parallel, said methodincluding the step of controlling the flow to said condenser means inresponse to three set-point levels of flow to said feedwater heatermeans, one being for no flow to said feedwater heater means, the secondfor flow only to one of said heaters, and the third for flow to both ofsaid heaters.

3. A method according to claim 1 wherein the flow to said feedwaterheater means is substantially the maximum permissible for optimum heatrecovery.

4. A method according to claim 1 wherein the control in response to flowto said feedwater heater means is locked-out when level in the flashtank means drops below minimum to avoid control of the flow to saidcondenser means in response to a high vapor flow to said feedwaterheatermeans.

5, A method according to claim 1 wherein said gen erator includes adeaerator in said main flow path, the method further including the stepsof pegging the deaeratorliwith auxiliary steam very early in thestart-up period an flowing all the flash tank drain flow to thecondenser means until the temperature of the fluid in the flash tankmeans reaches a predetermined level suflicient for flashing of the fluidin said feedwater heater means.

6. A method according to claim 5 wherein said temperature is about 200F. and the pressure in the by-pass line is about 1100 p.s.i.

7. A method for starting up a once-through vapor generator of the typecomprising a main flow path including heating surfaces, condenser means,deaerator means and feedwater heater means in series, a by-pass linebetween said heating surfaces and said condenser and feedwater heatermeans including flash tank means and drain conduit means between saidflash tank means and condenser and feedwater heater means, comprisingthe steps of:

initially in the start-up period flowing all the flash tank drain flowto the condenser means until the temperature of the fluid in the flashtank means reaches a predetermined level sufficient for flashing of thefluid in said feedwater heater means;

thereafter flowing the maximum permissible drain flow to said feedwaterheater means for optimum heat recovery depending upon the feedwaterheater means design limits;

controlling the flow to said condenser means in response to a flowsignal proportional to the flow to said feedwater heater means as longas the liquid level in the flask tank means remains between maximum andminimum points;

at levels below said minimum point locking-out said flow signal fromcontrol of the drain flow to said condenser means; and

at levels above said maximum point controlling the drain flow to saidcondenser means to maintain the flash tank level at or below saidmaximum point.

8. A method according to claim 7 wherein the feedwater heater meansdrain flow is controlled to maintain a predetermined level in the heatermeans and therefore pressure in the bypass line.

9. A once-through vapor generator start-up system comprising:

a generator main flow path including heating surfaces therein, condenserand feedwater heater means in series flow relationship with said heatingsurfaces;

a start-up bypass line between said heating surfaces and said condenserand feedwater heater means;

flash tank means in said bypass line including a liquid space and avapor space;

first and second drain conduit means connecting said flash tank meansliquid space to said condensing and feedwater heater means respectively;

flow control valve means in said first conduit means;

the improvement comprising control means for opening and closing saidflow control valve means;

first signal means responsive to maximum and minimum liquid levels insaid flash tank means;

second signal means responsive to the flow rate in said second conduitmeans;

both said signal means being operative to actuate said control means,the second signal means being operative when the liquid level in theflash tank is between said maximum and minimum liquid levels.

10. A start-up system according to claim 9 wherein said feedwater heatermeans includes two feedwater heaters in parallel of the shell and tubetype, said second signal means having three set points for alternativelyone, two or no heaters in service.

11. A start-up system according to claim 9 wherein said feedwater heatermeans is of the shell and tube type, further including means to controlthe liquid level in said feedwater heater means and thereby the pressurein said bypass system.

12. A once-through vapor generator comprising:

a generator main flow path including in series heating surfaces,condenser means following said heating surfaces, and feedwater heatingmeans preceding said heating surfaces, said feedwater heating meansbeing of the shell and tube type;

a start-up bypass line;

means connecting said bypass line to the main flow path at a pointintermediate portions of said heating surfaces;

flash tank means in said bypass line including a liquid space and avapor space;

said bypass line including a first drain connection between said flashtank means and condensing means and a second drain connection betweensaid flash tank means and feedwater heating means;

flow control valve means in said first drain connection;

stop valve means in said second drain connection;

flow controlling means opening and closing said flow control valvemeans;

means responsive to maximum and minimum liquid levels in said flash tankmeans transmitting a first signal to said flow controlling means whenthe liquid level in said flash tank means is above or below maximum andminimum points;

flow rate measuring means responsive to the flow rate in said seconddrain connection transmitting a signal to said flow controlling meansoperative when the liquid level in said flash tank means is intermediatesaid maximum and minimum points;

means to maintain portions of the heating surfaces upstream of saidbypass line at a high pressure; and

level control means for said feedwater heating means maintaining thebypass line at a pressure lower than said high pressure.

13. A once-through vapor generator comprising:

a generator main flow path including in series heating surfaces, heatsump means, and feedwater heating means of the shell and tube type;

a start-up bypass line between said heating surfaces and heat sumpfeedwater heating means;

flash tank means in said bypass line;

flow means to transmit during start-up a drain liquid flow from saidflash tankmeans to said feedwater heating means;

said flow means including flow control means controlling a rate of bleedflow from said drain liquid flow to said heat sump means to maintainsaid drain liquid flow to said feedwater heating means at a maxi mumpermissible level dependent upon design limits of the feedwater heatingmeans for optimum heat recovery;

said flow control means also being responsive to maximum and minimumliquid levels in said flash tank to maintain the flash tank liquidbetween said levels. said levels.

References Cited UNITED STATES PATENTS 3,021,824- 2/1962 Profos 122'406XR 3,183,896 5/1965 Lytle et al l22406 3,159,145 12/1964 Strohmeyer122-406 XR 3,286,466 11/1966 Stevens 122-406 XR KENNETH w. SPRAGUE,Primary Examiner

